The invention relates to the treatment of a waste liquid still containing organic and inorganic components which results from a fermentation process, e.g., from alcoholic fermentation, glutamic acid fermentation, organic acid fermentation, or others.
In recent years, water pollution in rivers and seas has become a particularly difficult problem for fermentation industries where big amounts of waste liquids, for instance, an alcohol distillation waste liquid are obtained as byproducts. For example, when liquors containing waste molasses and yeast are submitted to an alcoholic fermentation process followed by distillation of the resulting alcohols, such waste liquids have to be made harmless before they are released into the rivers and seas.
Conventionally, such waste liquid which contains organic and inorganic compounds has been submitted to a biological treatment such as a methane fermentation treatment or an activated sludge treatment. However, the resulting waste liquid has a brown color and still exhibits a BOD value (Biochemical Oxygen Demand) of 500 to 1000 ppm after such biological treatment, which will create secondary pollution. A known process for overcoming this problem comprises incinerating a waste liquid after concentration and recovering the ashes which are composed of inorganic salts such as potassium salts, calcium salts, or the like.
The recovered ashes include potassium compounds as a major component and also contain calcium, magnesium and silica, and therefore are effective as a fertilizer. However, the ash is obtained in the form of fine powder and, thus, has a tendency to be scattered when spread in the fields. Therefore, it is necessary to further treat the ashes in order to form a granulate. In addition to the minerals, a waste liquid from alcohol fermentation itself also includes organic components which are highly valuable as a fertilizer, such as crude proteins, amino acids and others, and therefore it has been highly desired to effectively use these components as fertilizer. However, the alcohol waste liquid exhibits strong caking and hygroscopic properties as it is concentrated and, therefore, it is difficult to achieve a concentration by mere drying. From a point of view of antipollution and profitable reasource utilization, an attempt has been made in recent years to produce a composition which is suitable as a fertilizer and is convenient for usage, in which the incinerated ashes are mixed with the concentrated waste liquid. Thereby, the adhesive property of the waste liquid is utilized to bind the ashes and to permit them to be granulated in order to reduce the scattering property of the ashes while at the same time the hygroscopic property inherent in the waste liquid as such or in the dried form is reduced. However, concentration of the waste liquid, as well as the incineration of the concentrated liquid and the uniform mixing of the concentrated liquid and the ashes is very difficult to perform in practice. The conventional process is normally such that a concentrated waste liquid is incinerated within a furnace, which is provided with a boiler. In the boiler which utilizes the heat of the combustion steam is produced and then this steam is used to concentrate the waste liquid within a multiple effect evaporator. In such a process, the incinerated ashes frequently adhere to the heated surface of parts of the apparatus such as the boiler tube and spoil its heat transfer performance and have a tendency to plug the flue, resulting in discontinuance of the operation. Furthermore, the multiple effect evaporator need to be frequently passed, e.g., by a chemical washing solution, in order to wash out scales deposited to the wall of it. The washing is, in some cases, required to be carried out after disassembling of the evaporator. Moreover, with the use of such evaporator it is difficult to concentrate the waste liquid to a high degree.